1. Field of the Invention
The present invention relates to a photo-receiver which is a combination of a photo-diode and a preamplifier, and more particularly to a photo-receiver which has a circuit for protecting an amplifier.
2. Related Background Art
A prior art photo-receiver of this type is configured as shown in FIG. 1. A cathode of an avalanche photo-diode (APD) 1 is connected to a biasing power supply (not shown) and an anode thereof is connected to an input terminal of an amplifier 2. Power is supplied to the amplifier 2 from a positive power supply and a negative power supply (not shown). When a light is applied to the APD1, a photo-output current is produced and it is amplified by the amplifier 2.
A voltage supplied to the APD1 by the biasing power supply is approximately 30 volts when the APD1 is made of germanium (Ge). It is approximately 80 volts when the APD1 is made of indium gallium arsenide (InGaAs). Because the voltage to be supplied to the APD1 is high, the following problem is encountered.
If the supply of the power to the APD1 by the biasing power supply is started after the start of the supply of the voltage to the amplifier 2 by the positive power supply and the negative power supply and before the fixation of the potential at the input terminal of the amplifier 2, the potential at the input terminal of the amplifier 2 rises at the moment of the start of the supply of the power to the APD1 and the amplifier 2 may suffer failure. This is explained below. At the time of the start of the supply of the power to the APD1, no charge is stored in a junction capacitance C.sub.APD across the terminals of the APD1. As a result, a potential difference between the terminals is zero and the high voltage supplied from the biasing power supply is applied to the input terminal of the amplifier 2 as it is. As a result, the amplifier 2 would suffer breakdown.